The present invention relates to method and apparatus for laminating a plurality of tapes or other elongate yielding plies.
The utility of structures fabricated by laminating a plurality of elongate, substantially continuous plies, such as tape plies, has been established. For example in the aircraft industry, high-modulus, lightweight laminated structures have been successfully used in airframe construction. In such an application, where minimal weight is not only desirable but in many cases essential, laminated plies of resin impregnated fibers are employed, to produce structural ocmponents which in many cases are preferably to lightweight metals, such as aluminum, or at least a suitable alternative thereto. The individual plies forming the resulting structure are usually provided in the form of webs or tapes, each tape being made up of a multiplicity of longitudinal, or sometimes cross-ply, fibers which may be either pre-impregnated with resin and wound on supply reels for later use, or drawn through a resin bath immediately prior to the combining and laminating stages.
For certain, advanced forms of these laminated structures, a particular laminating procedure known as a "pultrusion" process is employed. In such case, the tape plies are longitudinally combined and pulled as a composite unit through a heated die. The die which is similar to a draw die, compacts the tapes into a shape determined by the die cross section, while the heat simultaneously cures the resin. The process is usually continuous in the sense that the longitudinally aligned plies are substantially continuously drawn through the die, with the curing taking place while the material is within the heated zone of the apparatus. For high-modulus composite structures, the plies may be formed of resin impregnated filaments of glass, graphite, boron or various combinations thereof. while such materials may be supplied in the form of yard goods or wound tapes, the principal application of the present invention is to structures formed from elongate plies in which the length is many times greater than the maximum lateral or width dimension, as in the case of tapes.
A typical facility for pultrusion fabrication includes a tape feed system in which a plurality of rolls or spools of source tape are held and dispensed as needed to the downstream processing stations. From the tape feed facility, the plurality of tapes are guided into longitudinal and lateral alignment and combined, usually by overlaying one tape on another and pressing the confronting tape faces together. This latter process is sometimes referred to as a tape "lay-up" procedure. After combining the tapes in this manner they are pulled through the die, by a downstream puller facility, which may be provided by a "hand-over-hand" clamp or gripping mechanism which continuously or intermittently pulls the resulting structure through the pultrusion die. Rapid heating of the materials as they pass through the die is typically accomplished by dielectric or microwave heating, with the latter being preferred for high-modulus tapes.
In a pultrusion process the composite tapes or plies are pulled in a more or less continuous fashion through the die. However, other processes have been developed whereby the laminating step is effected in an intermittent or discontinuous process, sometimes called "step molding". An example of this so-called "step molding" process is disclosed in U.S. Pat. No. 2,977,630 issued to S. M. Blazer on Apr. 4, 1961. Other examples include a continuous laminating procedure as illustrated in U.S. Pat. No. 2,822,575 issued to R. W. Imbert et al. on Feb. 11, 1958, and another "step" process as shown in U.S. Pat. No. 3,236,714 issued to G. R. Traut on Feb. 22, 1966.
In general, method and apparatus of this type have involved the production of laminated structures having a cross section which is continuous throughout the length of the resulting product. However, in many cases, there is a need for laminated structures of this type which vary in cross section as a function of the structure length. For example, longitudinally tapered structural members are useful in the design and construction of airframes. In such case, a high-modulus laminated structure having a longitudinal taper, can be used for beams, spars, or stiffener caps, in which the magnitude of the load on the structural member varies as a function of its length. In such case, one end of the structural member may support a relatively large bearing or shear load, while the opposite member end has a relatively reduced load requirement. By positioning the relatively larger cross section portions of the member to receive and support the heavier load, the lighter load may be effectively carried by the relatively reduced cross section. It will be observed, that there is an improved weight-to-load ratio, in that the larger and heavier cross section is used only where required by the structural load. Additionally, there are situations in which a higher degree of flexure may be required for one longitudinal portion of the member as compared to another longitudinal section of the same member. This differential flexure or compliance may be advantageously provided by fabricating the laminated structure with one or more longitudinal tapers.
In general the formation of tapers and laminated structures is known, as indicated by U.S. Pat. No. 1,852,006 issued to H. A. Emery on Apr. 5, 1932; U.S. Pat. No. 2,827,412 issued to W. C. McKay on Mar. 18, 1958; and U.S. Pat. No. 3,581,698 issued to John U. Bete on June 1, 1971. However, these and other piror art systems and processes are not suitable for mass production of longitudinal tapers in a multiple ply, laminated structure formed from a plurality of elongate resin impreganted fiber plies.